Determine order and degree(if defined) of differential equation

$\frac{d^{2} y}{d x^{2}}=\cos 3 x+\sin 3 x$

$\Rightarrow \frac{d^{2} y}{d x^{2}}-\cos 3 x-\sin 3 x=0$

The highest order derivative present in the differential equation is $\frac{d^{2} y}{d x^{2}}$. Therefore, its order is two.

It is a polynomial equation in $\frac{d^{2} y}{d x^{2}}$ and the power raised to $\frac{d^{2} y}{d x^{2}}$ is 1 .

Hence, its degree is one.